Erosion and Sediment Pollution Control Program Manual.pdf

inside diameter (D) of the inflow pipe (Hathaway and Hunt, 2006). For channel flow,
the designer can use the equivalent pipe diameter. Riprap placed in the bottom of
the plunge pool should be sized according to anticipated inflow velocities. Use the
recommended riprap gradation in Table 6.6. (Minimum R-3)
iii. The vertical separation (Y) between the inflow (pipe or channel) and outflow (weir)
inverts should provide sufficient freeboard to allow for energy dissipation. At a
minimum, Y should be equal to half of the inside pipe diameter (D). Where freeboard
is not possible, there should be a minimum 6-inch drop per 10 linear feet of plunge
pool length (See Profile View of Figure G.3).
iv. The level spreader should provide sufficient length (X) between the inflow point and
the outflow weir to avoid overtopping. If the distance (X) does not meet this criterion,
the potential for short-circuiting will increase significantly below the inflow point. In
this situation, the level spreader can include a raised middle section down slope of
the inflow point to prevent overtopping and to direct flow laterally. The crest of the
raised section should be equal to the crown elevation of the inflow pipe. (See Plan
View of Figure G.3)
v. Recommended values for X: (See Figure G.4)
If Y > ½D, then X = 3D (Minimum 8’)
If Y < ½D, then use Figure 9.4 to find X. (Minimum 10’)
Figure G.4. Variation of Plunge Pool Dimensions.
b. Accommodate draining the plunge pool (surface discharge):
i. To preserve infiltration capacity, the underlying soils should remain undisturbed,
uncompacted, and protected from heavy equipment.
ii. If standing water in the plunge pool is a potential issue (e.g., mosquitoes, icing, etc.),
the level spreader should include a drainpipe with a discharge point located in a wellvegetated
area and/or include soil stabilization measures, as appropriate. (See
Figure G.3) A 2-inch diameter drainpipe is recommended with a minimum 1%
slope. Where possible, installation of the drainpipe should extend laterally so that
the discharge point is located outside of the flow path of the level spreader. Where
this is not possible, installation by trench method should be avoided since it may
create a preferential seepage pathway for surface flow and erosion. In certain
instances, the drainpipe (ductile iron) may be driven horizontally with a
sledgehammer.
363-2134-008 / March 31, 2012 / Page 436